The present invention relates to a data communication apparatus and, more particularly, to a data communication apparatus that processes an interruption caused by a third station during communication of data.
This is a type of communication system in which image data is exchanged between facsimile apparatuses, or information processing apparatuses including personal computers, are connected together by a telephone circuit to exchange data. In such a communication system, during the communication of data between two stations, a circuit connection request is made from a third station other than the transmitting and receiving stations. A facsimile apparatus processing such a circuit connection request, is disclosed in Japanese Unexamined Publication No. Sho. 62-216584.
In this facsimile apparatus, when a circuit connection request signal is received, the present communication is temporarily suspended and the telephone number of the third station requesting circuit connection is stored in memory. The data communication, which has been suspended, is resumed, and after the completion of this data communication, the stored telephone number is automatically dialed to call the third station.
Further, there have been proposed a facsimile transmitter and a data communication system (Japanese Patent Unexamined Publication Nos. Sho. 64-78575 and Sho. 64-86660) wherein, when an interruption such a circuit connection request is received, the present communication is immediately suspended and switched to communication with the third station.
In the above transmitter and communication system, when an interrupt signal from the third station is received during data transmission or when an operator instructs an interruption during data transmission to execute an interruption processing, the circuit is cut off to suspend the data communication. After the execution of a given interruption processing, the data communication, which has been suspended, is automatically resumed.
As the above circuit connection request signal, a "catch phone" signal used in the service provided by NTT (Nippon Telegraph & Telephone K.K.) is known. In the "catch phone" service, when a circuit connection request is made from the third station, a call signal ("catch phone" signal) that inform the operator of the circuit connection request is sent out from an exchange. The "catch phone" signal is a signal of frequency 400 Hz which is interrupted at predetermined intervals.
In addition to the "catch phone" signal, various call signals are known, which are selectively used in accordance with the type of exchange used. For example, a call signal used in ATT, U.S.A., is a hybrid signal which comprises two signals of 480 Hz and 620 Hz and is output at 60 IPM. In addition, voice or music has recently been used as a call signal and a holding signal that is generated when the called party responds to the call signal.
When a "catch phone" signal comes in to inform the operator that a circuit connection request is being made from a third station, noise may enter the data which is being transmitted due to the "catch phone" signal. For example, in a facsimile communication, picture information is transmitted by use of a frequency band of 0.3 to 3.4 kHz. Therefore, the frequency (400 HZ) of the "catch phone" signal is included in the occupied bandwidth of the facsimile communication, so that the transmitted picture information may be disturbed by the "catch phone" signal.
When the present communication continues without responding to the call from the third station, it is likely that all the picture information received while the call is continuing will be adversely affected by the "catch phone" signal.
When the apparatus or system disclosed in the above-mentioned publications (Japanese Unexamined Publication Nos. Sho. 64-78575 and Sho. 64-86660) is employed, the circuit is cut off and it is therefore possible to minimize the disturbance of the information. However, the prior art involves some problems because the present communication is suspended on every interruption.
More specifically, during the suspension of the communication, a call from another station may come in the remote station which is kept suspended in the middle of the communication. In such a case, a great deal of time may be needed for redialing, or it may be impossible to connect the circuit with the remote station for a while. Thus, there are cases where it is better not to suspend the present communication unconditionally when a "catch phone" signal comes in.
However, no consideration has previously been given to not suspending communication in the conventional apparatus and communication system. Thus, the prior art suffers from the problem that the circuit is switched over instantly when an interrupt occurs.